Hacker News new | past | comments | ask | show | jobs | submit login
Aviation Is on a Low-Carbon Flight Path (scientificamerican.com)
97 points by flip8 5 days ago | hide | past | web | favorite | 247 comments

Electric aviation for aircraft larger than 4 passengers is extremely hard. Batteries don't have the energy density needed, and fuel cells are barely at the prototype stage for aviation. ARPA-E has been looking at this for while, see slide 6 in [1] for an overview of payloads by energy storage medium. If you read a little more through the deck you'll see there are also issues with power output of electric motors in this application.

I'm much more optimistic about low-carbon jet fuels. Biofuel options here have already been trialed successfully. And some new synthetic fuel companies [2] that use direct air capture CO2 are looking promising.

[1] https://arpa-e.energy.gov/sites/default/files/Grigorii-Solov... [2] https://en.wikipedia.org/wiki/Carbon_Engineering

It's also hard because batteries don't get lighter as they deplete, so you're landing a plane with the exact same weight as what you took off with, meaning you need much more rugged (and heavy) landing gear. This is extremely significant when you consider that a fully loaded 747-8 is 970,000 lb, of which up to 326,000 lb is fuel (per Wikipedia's numbers).

It's a hard problem, and most proposed solutions are extremely novel and risky, like having an all-battery tugboat that tows the plane up to cruising speed/altitude before returning to the airport to charge or get a battery swap— obviously bad because risky, also adds 50% to runway capacity to land all the tow planes.

Aren’t planes already designed for landing at full weight for cases where the plane needs to turn around and land where it departed?

Yes, but they are only intended to use it in rare emergencies. It stresses the airframe components and reduces the life of the aircraft. To build an aircraft that lands heavy every time you need a much stronger undercarriage and frame.

and how often are planes fully loaded with fuel? A plane only needs enough fuel to reach the next destination. Saves the weight and money.

Some planes are capable of that and some aren't. Some have fuel dumping systems to get weight down.


Many are equipped with fuel dumps. Others have mandatory “overweight landing inspections” prior to further revenue flight.

Your car can run past redline, it just shouldn't all the time.

Just drop them before landing... There could be a batteries landing place where they'd be collected.

Is it too crazy?

It turns out that aircraft have a very sensitive centre of gravity that needs to be close to the centre of lift. If you were to design a "battery bomb" you'd have to carefully engineer the pre- and post-bombing centre of mass and ensure that your control surfaces can respond adequately. In addition, you'll need bomb-bay doors and associated mechanics which adds mass and complexity. All of this takes a toll of efficiency, cost and reliability. However, it's an intriguing idea, especially if you use steerable parachutes that can neatly fly themselves back home, maybe all the way into a recharging slot.

Since a big chunk of the power is expended on takeoff, the ideal would be to drop part of your battery then rather then waiting until after landing. But either way, you're stuck with the significant complexity and expense of having to prepare a parachute and landing gear specifically for the battery on each flight, plus having to collect and charge those batteries.

Accepting that we're just throwing out interesting-sounding ideas, I wonder if having a powered runway rail (or something of the sort) to provide takeoff power could work. With that you wouldn't have to worry about dropping anything.

But I do wonder how much power is actually expended at takeoff versus climbing or cruising flight. According to [1], only ~19% is used for both takeoff and climb, so electrifying the runway to save the fraction of the 19% used while in contact with the ground is probably not worth it.

[1] https://aviation.stackexchange.com/questions/47262/how-much-...

Could use an electromagnetic rail to slowly but surely accelerate and launch it up to to cruising altitude https://en.m.wikipedia.org/wiki/Rocket_sled_launch

SupersonicScrub's comment (a sibling to yours) says that the engine can be significantly more efficient if it doesn't have to meet the power demands of take-off. Their link says "at least 30%" fuel savings on a one-hour flight (although "the fuel savings will partly come from the latest gas turbine technology"). So maybe the rail system you propose isn't such a crazy idea. Also, it's not totally different in concept from an aircraft carrier's catapult; hopefully gentler...

>Also, it's not totally different in concept from an aircraft carrier's catapult; hopefully gentler...

Nah. If someone can't handle a few g's in takeoff, maybe they shouldn't be flying, and should should stay home, or maybe in a nursing home. I know it sounds mean, but seriously, think about it: aircraft transport isn't always comfortable. Turbulence happens, and it can be extremely rough at times, even injuring people severely if they're not belted in. If someone is too frail to handle being launched by an aircraft carrier catapult (which, remember, also launch those big AWACS planes, not just small fighter jets), then they're not suited for handling turbulence either.

Also, don't forget, the latest generation of catapult technology is actually pretty gentle. The USS Ford's "EMALS" catapults are electromagnetic, rather than steam, and one of the big stated benefits of this is that it's gentler on the airframes than the old steam catapults. It's still going to subject the pilots/passengers to several g's, but probably lower peak acceleration than the older technology did.

There's a better electrically driven solution that takes advantage of the fact that take-off and climb-out requires the most power.

Maximum power requirements come from take-off and climb-out, so the engine size is designed around the max take-off power. This means that the engine is over-designed and less-efficient for the majority of the flight.

By implementing a hybrid engine, the gas-powered component and the electric powered component work together to provide take-off power requirements. By the time the aircraft reaches cruising altitude, the batteries are dead, and the gas-powered component works alone. This allows the engine to be designed for the cruise power requirements, which results in a much more efficient engine. UTC is currently experimenting with this concept.


Yes it's not carbon neutral, but it's a step in the right direction.

Not just for take-off, but for the possibility of finishing the take-off with one engine failure. That's quite a bit of spare capacity.

However I also wonder why, if electric motors are a solution here, there hasn't been a comparable fuel-powered extra-takeoff-engine. Why would this not have the same benefits? Or are extra engines simply too complicated mechanically unless they are motors?

(The liked article is about modifying a Dash 8, which is a turboprop, and perhaps it's easy to have oversize propellers for takeoff, and just connect a motor to the same gearbox.)

Probably because a jet engine costs like $30MM. It may well be that the incremental cost ($, weight, complexity) of adding an electric booster to an existing engine design is much more practical than just adding more engines.

You still need enough power to make a late go-around at the destination, in case of problems due to weather on final approach, arrival spacing, or a problem on the runway. I've been on planes that had to go around from low altitude three times, so it's not even that rare. Either the engine needs to be large enough to generate full take-off power, or you need a large margin of battery power that will never be used on an uneventful flight.

> take-off and climb-out requires the most power

Military aviation has dealt with this in various ways:

1. Carried aloft by a larger plane

2. Towed aloft be multiple other planes

3. Jet assisted takeoff

4. catapults

5. drop tanks

6. mid-air refueling

Wouldn't this also mean that full engine power is unavailable in the event of an emergency? Seems like this solution makes edge cases more likely to be fatal.

Perhaps you could trail a long unwinding power cable behind you which disconnects when fully extended.

Heck, it might be doable to just lay the power cable in a permanent route on the ground and use electricity to propel the craft along that route! You wouldn't even need wings!

Aircraft carrier catapults do that. When launching the aircraft, all the energy is expended from the ground. Looking forward to such gigantic machines on airports, how much they would charge per flight, and problems like terrain slowly moving due to the gigantic forces at play.

Catapults accelerate aircraft to the point of generating enough lift for powered flight. They allow you to take off from a short runway like a carrier but don't get you to altitude. You can get a sense of how much power is being used to reach cruising altitude by listening to the engines on take-off. They don't fully throttle back until you reach a high enough altitude to permit level flight.

>Catapults accelerate aircraft to the point of generating enough lift for powered flight.

Yes, but the reason they're used is because it's infeasible or too expensive to build an aircraft carrier long enough to have jets reliably take off from them without this extra boost. Even so, those catapults are fairly short: notice they're only a fraction of the length of the carrier.

They're not used in regular airports because they're not absolutely necessary, the way they are on aircraft carriers. It's more expense, and planes can do without them. However, there's a cost to this: the planes need bigger engines and more fuel to take off reliably. And bigger engines are less efficient overall (as in cruising). So there would be a significant fuel savings if you could count on having catapults on the ground to help get the plane to takeoff speed: you could downsize the engines, and carry less fuel, saving a lot of money overall per flight (not to mention all the carbon pollution).

Finally, the extra g-forces shouldn't be a big deal. There's high g-forces in a carrier launch because the catapults are very short. And until recently, they were steam powered. This needn't be the case with commercial aviation: they can use modern electromagnetic launchers, and they can make the catapults very long (as long as much of the runway), to have lower peak acceleration. Basically, the planes and passengers would not need to experience higher g-forces than they already do; they'd just be getting them from the catapult instead of solely from the engines.

Good point - if you watch carrier launches, the planes actually fall a little as they go off the end of runway because they aren’t lifting yet. But presumably with a longer runway like you have at an airport, you could get to higher speed and truly liftoff.

But having also launched from a carrier myself, I wouldnt want that g force everytime I fly commercially.

> the planes actually fall a little as they go off the end of runway

If I was a carrier pilot, I doubt I'd ever get used to that!

I would love to launch from a carrier. Was in the Air Force but not flight crew. Saw a lot of great aircraft there...from the ground. :)

> Looking forward to such gigantic machines on airports, how much they would charge per flight

It's actually an interesting supposition, because whether it's gasoline or electric, it should be cheaper and more reliable to run a ground operation than to pack fuel that is expelled before anything meaningful has happened.

Or build a several kilometers high tower, using either Space fountain or floating spheres.



What happens when it mechanically fails and doesn't disconnect? Have a 2 mile long cable being drug behind a 200mph flying machine?

That seems quite dangerous - to both the aircraft and everyone onboard, as well as to anyone on the ground in the path of this cable.

What happens when the plane mechanically fails? Having 600 tons of metal raining down on a population sounds quite dangerous.

> What happens when the plane mechanically fails? Having 600 tons of metal raining down on a population sounds quite dangerous

Simply put, that's just not how aviation works.

Mechanical failures aren't uncommon in aviation, but for an aircraft to not be able to divert and return to the ground, a lot has to go wrong all at once. It's infinitesimally rare for all systems to fail at once on an airliner. There simply isn't a problem of airliners dropping out of the sky, ever. They're incredibly robust machines.

However, with your idea, we'd be relying on a single system to prevent catastrophic failure. That's a massive step backwards for aviation.

That doesn't even begin covering the problems with delivering enough electricity over this cable to power an airliner at takeoff... That cable would need to be a very thick gauge of wire and very heavy - causing all sorts of it's own problems.

You can have your doubts but I'm off to the patent office tomorrow.

It wouldn't be viable for flights over oceans but I wonder if having batteries that eject at 95% exhaustion and then fly/glide themselves to a collection point with the last 5% of energy would work. There are too many variables for me to even do cocktail napkin math on it but with ever increasing drone tech, perhaps it would be possible.

Hard to say if giving the battery its own avionics is better or worse than equipping it with a parachute or steerable glider. Either way, it's the kind of thing that SpaceX decided was worth trying for the sake of fairings worth a few million apiece. Asking them how it went is probably a good place to start.

Regardless of the specifics, it's obvious that any of these schemes is a logistical and technical nightmare compared to "taxi up to the terminal, plug in the refueling hose while people and cargo are loaded onto the plane." It's unlikely that the private sector will be too enthusiastic about investing in anything like this without heavy incentives.

I wonder if their are a pair of liquids that could be used to make a flow battery and that would be safe to dump they are expended?

The liquid is gasoline, and you can make it carbon neutral. This whole effort toward battery powered aircraft is misguided: technically, hydrocarbon fuels work better than anything else. What you want to do is make carbon neutral hydrocarbon fuels.

The real advantage of electric airplanes is simplicity of electric motors, better aerodynamical properties of a plane using many small propellers, and possibility of vertical takeoff.

Ideally when we have kerosene based fuel cells we could have small electic aeroplanes that will be much easier to control, and would allow more frequent direct flights, instead of having to collect 100s of people together make them wait several hours, and drop them at an inconvenient location.

Sure. I have no objection to electric power trains for aircraft. But at that point, we're not talking about eliminating carbon emissions anymore, so the climate angle in this article ceases to be relevant.

In a word, yes. Dropping the batteries out of an airliner in a way that left the batteries recoverable and reusable would add a tremendous amount of weight. It would also add a tremendous amount of danger. Neither of those things are attractive to a manufacturer.

However, reducing the cost of fuel by 1/10 would be very attractive to basically all airlines, so the manufacturers may very well step up for it. Bombs away!

The reason Boeing developed the 737MAX was a 15% fuel savings. Pretty darned attractive.

Here is what happens when you've got a tank rolling around downstairs:


Jettisoning 200,000 lbs of batteries would be even worse.

I'm not sure that's relevant. Said tank was an uncontrolled and unexpected movement. Military air transports are certainly capable of doing controlled air drops of tens of thousands of pounds.

Batteries tend to have chemicals that are both fire and corrosion hazards. Subjecting their cases to the stresses of repeatedly landing (even on a padded catcher) could lead to containment problems. You don't want that on an airplane.

Apparently they do actually get lighter, but not usefully so.

Why is that? I’m not a battery expert but it would seem all that charging a battery does is move electrons in the cells around, no?

e=mc2 I think - there’s a Tesla forum thread about it, and Quora articles and things if you look. I’m not a physicist.

The mass depletion is less than a milligram...

Yeah I read the same threads - that’s what I was referring to.

Chemical bonds have (often negative) mass. A chemical reaction will break and/or form bonds. Thus it slightly changes the mass. In a battery there will be chemical reactions happening at the anode and cathode. One reaction produces electrons and another absorbs them.

Why wouldn't you just ride on the tugboat?

It doesn't have the range for useful travel. It's designed to get the vehicle up to speed, using almost all its energy, and then drop away. Think of the first stage of a rocket.

> Electric aviation for aircraft larger than 4 passengers is extremely hard.

There is at least one airline transitioning to only electric aircraft already [1]. It's one that focuses on short range flights. And as energy density continues to increase, the range and applications of all-electric aircraft will too.

[1] https://electrek.co/2019/03/26/harbour-seaplanes-electric-ai...

Ignore the PR fodder and go run the actual numbers. The actual range/endurance/payload capability of these aircraft is a tiny tiny fraction of their Jet-A/100LL counterparts. It will eventually get better, and a lot of organizations are smart to keep that in mind, but we're not there yet and won't be for a decade at least.

I specifically mentioned that the range and applications of this initial version is very limited. I was mostly just pointing out a company (with a very unique service) that is at the point where it's possible. It won't make its way to major carriers for a long time (i think a decade would be quite optimistic, given lead times in aviation)

Solid Oxide Fuel Cells are reaching 2.5kw/kg power densities, Electric motors are reaching 15kw/kg power densities. Combined, that is already better than current jet engines. With cogeneration, SOFCs are reaching 80-85% efficiencies, double that of jet engines. Liquid hydrogen has an energy density of 39kwh/kg, compared to JP8 at 13kwh/kg, but SOFCs could run on pretty much any hydrocarbon. They don't even need cryogenic storage, because fuel consumption rates would be higher than evaporation rates.

All the pieces exist for fuel cell powered jets. They've already got the performance levels needed. All they need is to put them together into a cohesive package, and put a shit ton of effort into reliability engineering.

> Combined, that is already better than current jet engines.

Wikipedia lists the GE90-115B at 10kW/kg, four times better than your SOFC. Did you just combine motor and fuel cell to get 17.5kW/kg? That would be hilarious.

> With cogeneration, SOFCs are reaching 80-85% efficiencies

That's creative accounting. You're counting waste heat as if it was useful.

Cogeneration isn't much use on a plane...

Granted a little heat for the cabin is handy, but the rest is largely useless.

I meant in the context of energy conversion. Solid oxide fuel cells produce high grade heat (~800C) that is perfect for conversion via organic rankine cycles (which will convert ~30% of that heat to electricity) or supercritical co2 brayton cycles (~50% heat to electricity with extremely high power densities[0]).

Even without heat capture, SOFCs run at 60-65% efficiency, which is probably good enough.

[0] https://www.ge.com/reports/call-ecomagination-ge-building-co...

Energy density of jet fuel is 45MJ/kg. Fuel is a significant portion of the weight on the plane. Requiring a 10x+ increase in fuel weight is a non-starter no matter how light the engines.

If you're measuring in MJ/kg, hydrogen is 141MJ/kg. Hydrogen, in terms of weight, is absolutely better than jet fuel, not worse. It's not very dense (kwh/l) though, and that could be a problem.

Even for aeroplanes with just two people, the most usable, all-electric endurance I've seen is about 1 hour[1]. Just about usable for training (mostly circuits around the airfield), but not much else.

[1] https://www.pipistrel-aircraft.com/aircraft/electric-flight/...

Useable in the loosest possible sense of the word since charging it would then take a few hours. By which point lack of daylight or a change in the weather would probably mean no more flying for the day. And even if that could somehow be made to work (perhaps with swappable batteries), it ignores the fact that anyone trained in flying an electric aircraft would be poorly equipped to fly a conventional light aircraft with issues like carburettor icing, shock cooling, leaning with altitude etc.

I imagine there'd have to be swappable batteries, but you're absolutely right that you'd be missing out on learning all the other aspects of flying. Although by the time electric aircraft become mainstream for training I'd hope we'd seen the end of carbs!

I thought these were in use at flight schools already. With spare batteries being charged on the ground. Perhaps you can't practice quite everything, but as a cheaper way for people to get some of the hours it sounds great.

I had a bit of a look, I found one flight school that had committed to buying some electric aircraft and had apparently trialed them before. No real indication of how it works practically. Bearing in mind that a Tesla battery pack weighs 500kg (1200lb) or so. Changing them over wouldn't be a simple operation.

Looks like this is what I was remembering:



They have electric planes, and they have flight schools, but whether the two actually intersect or not isn't immediately clear.

What doesn't make sense is to use number of passengers as the figure of merit, here. Technically, that's no limit at all. Aircraft scale up just fine from 2 people to 20 to 200 people.[0] "Power" is even less of a concern: fundamentally, electric motors are just as high performance as turbine engines.

The appropriate figure of merit is range and specific energy (i.e. useful energy stored per unit mass), and that IS a limitation, but range of up to 500 miles is feasible for pure electric using existing batteries and structure and aerodynamic efficiency tech. Double or quadruple that is possible with cleverness and more exotic approaches. Even long-haul is possible with certain approaches to pure electric flight (either lithium-air, which has just as high theoretical useful energy as something like gasoline, or staging).

The only way "large aircraft" or "high power" is a limitation is just the speed at which things can be certified. Large passenger aircraft cost billions to bring to market, so companies are super conservative about it. But this isn't a fundamental technical limit of electric flight.

[0]Technically, there are structural scaling laws which detract from efficiency as you get bigger, but they don't become strong until you're very large... And they're countered by perhaps even more important positive Reynolds Number scaling effects, i.e. your lift to drag improves as your scale increases.

> staging

I'd like to see a prototype of an electric airliner which can fly forever, but every half hour or so a car-sized battery with little stubby wings flies up and docks into the back of the plane.

Not sure about the utility of an airliner flying forever unless you've got passenger pods attached to the flying batteries :D

...And now we've invented zeppelins again

Don't forget that most long-haul planes can't land at their MTOW - so the batteries need to be scaled down apropriately.

Or more attention must be paid to engineering landing gear and structures to handle high landing weights. One must approach the problem with an eye to identifying solutions or we'll never get anywhere.

There's more to it than just structural integrity - the heavier plane is generally less safe in an emergency, and despite some ideas proposed around you can't "get rid" of takeoff-power engines unless you drop Go-Around capability.

There are other ways to ensure safety during an emergency. Not having flammable fuel on board can help a lot (and requires a non-flammable electrolyte to be developed for lithium batteries, which is already a thing for solid state batteries). Additionally, not all civil aircraft even have go-around capability. Sailplanes don't, for instance, and their safety is approximately the same as other general aviation aircraft.

We'll have to get rid of the fiction that we're going to be able to just plop in electric motors and batteries in existing civil transport vehicles without any change in architecture or operations.

Are you going to get into a transport airplane that can’t go around? I’m not going to. I’ve been in two low level go arounds on commercial flights in VFR conditions (good weather) and one missed approach (bad weather) in the last two decades. Is that three times that I would have crashed? Maybe not; Delta probably could have stuck the downwind landing in Vegas and USAir probably could have stopped before the airplane that was slow in clearing at BWI. Delta into AMS in the snow and fog probably would have been a crash if they couldn’t execute the missed.

Sailplanes are probably not the right model to go flying IFR to minimums, something that airlines do regularly.

If it was engineered to be compatible with no-go-arounds safely, then absolutely I would. I've done it.

Or you rethink completely the problem, and beam the energy to the plane.

This is more on an engineering challenge, unlike much better batteries which will require a fundamental scientific breaktrough.


Beam propulsion is impractical for aviation due to weather effects and line of sight. It's more than an engineering challenge, it just won't work (except for maybe a few limited cases of very short range flight in clear conditions).

You can beam from over the clouds (beaming antenna on a tethered balloon or even drone), and have a huge line of sight to boot.

It's an absolutely huge engineering challenge, but at least it doesn't require a discovery worth of the Nobel prize in chemistry (which I guess much improved batteries would warrant).

Having a huge line of sight doesn't solve the problem. The receiver has to be oriented roughly perpendicular to the incoming beam in order to get any reasonable efficiency. And the receiver has to be large in order to account for beam dispersion and aiming inaccuracy. For aerodynamic reasons the receiver can only be on the top or bottom of the aircraft and can't be angled much. So therefore the power transmitter must be close to directly above or below the aircraft.

Battery energy density is steadily improving every year. The 2019 Nobel Prize in Chemistry went to the inventors of the lithium ion battery.


I know some very smart people who are working on this. http://el-sky.com/

I wish them luck but I doubt they'll find a magic solution to beaming power through clouds.

"Electric aviation for aircraft larger than 4 passengers is extremely hard"

To be honest, the worst parts of flying are usually all those other passengers anyways. If you could scale up a fleet on-demand small electric planes. Flying would improve. If you account for all the waiting time, it might be just as fast too.

That would also make point to point flights easier to coordinate. It might not make sense to have a direct commercial flight from Birmingham, Alabama to Fort Collins, Colorado with today's aircraft but if you only needed to fill four seats, such pairings might work. Not sure how much quieter an electric airplane would be but if it is significantly quieter, smaller airports spread out in greater numbers would also make point to point more viable.

Four seater aircraft have been around since the first decade of flight, and I'm not sure batteries are going to solve the cost and logistics issues of using them as taxis, except in edge cases where there's a mountain in the way.

Flying anywhere near as far as from Birmingham, Alabama to Fort Collins, Colorado is also an exceptionally hard problem for electric aircraft.

This is the same for the train and shipping industries. I’ve always thought those vehicles had ample opportunities to capture solar rays and wind since they’re out in the open all the time, but we simply don’t have batteries and engines powerful enough yet.

I mean, basically every train engine you'll see that isn't steam is powered by electric motors. The diesel engine just makes electricity for them, and is not used to directly drive them via transmission like in a car or truck.

Could we use Hydrogen that is generated sing clean electricity or is too dangerous?

It's possible, but even liquid hydrogen is not very dense and requires a lot of insulation.


Liquid hydrogen has about 4X the specific energy of Jet-A. But yes, the system as a whole isn't great once you consider what it takes to keep the H2 liquid...

Energy density per unit of mass is about 3x in favor of hydrogen. Energy density per unit of volume is about 4x in favor of Jet-A.

Sorry you are absolutely correct. Volume matters a lot, maybe almost as much as weight, for aircraft design.

> Batteries don't have the energy density needed

... today.

There's a lot of money in batteries these days.

But from what I gather, it's not just about the batteries, but also about coming up with a design that takes advantage of the possibilities offered by simple electric motors.

I challenged myself to go a year without flying for climate reasons. I thought I would hate it. Work, family, and lifestyle depended on it.

It turns out it was one of the best things I've done. Experience taught me what I never could have imagined -- how much my life improved. Like dropping Facebook on steroids. What I expected to miss I got more without flying, plus improved relationships, community, and connection.

A few months in I decided to go for a second year. I'm 4.5 years in now. I may fly again, I don't know, but I want to less and less.

EDIT: Amazingly, my TEDx talk on this experience went live minutes ago: https://youtu.be/sTYiHr1lu10.

This doesn't seem very hard to believe. Flying is a miserable experience.

If you're a die-hard reader and always have a book in hand, it's actually not that bad. You just happen to stand in line to read, rather than lounge or lie down to do so. Granted, obtaining and arranging accessories (pillows etc.) so as to sit comfortably in the seats does take some trial and error.

Or I could just stay home, and read in a more comfortable seat...

I try to avoid flying as much as possible but purely because I love driving across long distances. I don't particularly dislike flying and for international trips there really is no other alternative.

I found not flying a better alternative.

Everybody tells me what I would have said before, that it's impossible to avoid, which is why I pointed out how the experience changed everything. It turns out many things were possible I didn't think were.

Recently there has been some talk about banning air miles programs and actually flipping the incentives. Because you have to fly under your government ID it wouldn't be out of the question for people who fly a lot to pay more per mile in taxes.

I've been thinking about this a lot trying to root out the unintended consequences of such a plan, but I can't think of any.

Wouldn't a carbon tax on the source, when fossil fuels are extracted, be more effective?

You'd still be paying more for longer and more frequent flying, but you'd also be hitting personal jets harder than full flights, less fuel efficient planes and flight paths against better ones - and it would be reduced even further if an airline managed to electrify.

Although that still ignores the additional warming effect of radiative forcing caused by flight.


The proposals i've seen are nonlinear. Each person gets, say, one flight a year with no Frequent Flyer Tax. Your second flight is taxed at 100 dollars. Your third, 200 dollars, Your fourth, 300 dollars, etc.

The trick is to pitch it so that you don't have much impact on the 85% of the population who do 30% of the flying, but you strongly discourage the 15% of the population who do 70% of the flying [1]. Partly as a practical measure to ensure it's not so unpopular it gets repealed, but partly because those 15% are probably capable of making changes to their lifestyle or way of working that will drastically cut flights, whereas the other 85% are flying where they have to, to see their family at christmas or whatever, or for their one summer holiday.

You couldn't do that by taxing fuel, or aeroplanes, or anything other than individual people flying.

[1] https://fullfact.org/economy/do-15-people-take-70-flights/

That's a neat idea, but aren't most frequent flyers doing it for business reasons? In which case the business will pay the tab for the tax increase, most likely.

Businesses have a wide latitude to spend on aviation because business travel done right is incredibly lucrative.

The wrinkle I see with escalation of taxes per person per year is while my company would happily pick up the tab for work travel, I’d be upset if my Christmas trip home cost me thousands more because I’d done a handful of business trips.

Then that sounds like escalating taxes would be an extremely effective incentive for behaviour change, despite the fact that employers will be picking up most of the costs!

Or equally likely an extremely effective way to encourage private driving and private flying, both of which are less energy efficient and safe than long-haul airline flights.

Not to mention creating confusion and hassle for travelers, whose ticket price can’t be calculated and paid in advance (because you don’t know how many trips I’ll have taken before that flight), or who share a common name.

one way would be to make it revenue neutral as in take the money from frequent flyers and give it back as tax refund. then people will want more of it.

I can see a few with flipping incentives - flight is a subsituable good and a fuzzy one. While not encouraging it for its own sake is fair the alternatives may be worse.

I know of one case where a coworker ended a business trip by taking vacation days and a several day cross Atlantic cruise ship home which was likely worse than even a plane trip to an island as it didn't involve hauling essentially an entire luxury hotel thousands of miles.

I suspect a better idea for encouragement if anything would be subsidizing efficient clear neccessity/better than current baseline transit. Like say if living closer to work was subsidized along with public transit - both replacing or shortening automobile commutes.

This is a terrible idea. Some people have no choice but to fly. Say what you want but air travel is efficient.

Frequent and cheap air travel is a recent phenomenon, we can abandon it without going back to the Stone Age.

Who has no choice but to fly three or more times a year? Do they really have no choice?

I am reminded of an acquaintance of mine who once told me he had no choice but to drive, because he lived in a little village with one bus a day, but commuted into London. What he had overlooked is that he'd chosen to live in this village.

Pretty sure it doesn't take a literal ton of fuel to move me across Europe in a train. In an airplane, it does.

Depends on the plane and the load factor. Assuming a conservative 80 mpg per seat a flight from Moscow to London would consume the following to move your tail between locations:

1554 miles / (80 miles/gallon) * 6.8 lbs/gallon = 132 lbs

I was actually a bit surprised how little it is. It gets better in newer aircraft assuming all seats are taken.

[1] https://en.m.wikipedia.org/wiki/Jet_fuel

[2] https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft#Mediu...

Time is more valuable.

Indeed it is.

There'll be time after us.

You aren't against pricing in more of the externalities though, right?

For example, a round trip flight between NY and London is said to melt 3 cubic meters of arctic ice per passenger. So it wouldn't be unreasonable for airlines to be required to offset that in some way?

If so, then we just disagree on how to structure the tax but mostly agree on the need to reduce damaging flights.

What remarkable luck they were born after planes were invented!

You might actually see a huge lift in private aviation as I don’t have to show government ID or otherwise check in with the government for those flights (provided they take off and land in the US). You can’t even necessarily track the tail numbers of the airplane if it operates from non-towered airports away from our largest 150 or so airports and under 10K feet (so piston airplanes).

You would also displace a lot of travel to private autos, with a decrease in safety/increase in fatalities and typically an increase in fuel consumption when comparing a single occupancy car to a fairly full flight. Plus, if I’m going to be driving a car 500 miles pretty regularly, I’m going to splurge for a nice, big, comfy car and it’s not going to be an electric. Since I currently fly rather than drive on road trips, today I can drive a small electric as my daily driver.

> I've been thinking about this a lot trying to root out the unintended consequences of such a plan, but I can't think of any.

Record keeping, administration, etc.

You could replace it all with simple calculations:

Landing Tax: (tax rate) * (landing weight) * (distance traveled within US airspace)

Takeoff Tax: (tax rate) * (takeoff weight)

Record keeping is already in place for the air miles programs. It's a solved problem.

The two taxes you listed do nothing to make people who travel more pay more per mile than somebody doing their one flight a year to Disney.

What's the motivation behind making it a progressive scale per mile?

Is there a reason the Nth mile should be taxed more than the 1st when the environmental impact of the Nth mile and the 1st are the same?

In fact the impact of the 1st is substantially greater, because of the energy required for takeoff.

And since takeoff weight is linked to both passenger count & traveled distance (amount of fuel), it too accounts for distance!

I'm pretty sure that there are certain airlines that are nearly unprofitable from their actual airline operations, but only eke a decent profit due to their airline miles programs (specifically their credit card rewards programs). So it seems like you are suggesting that we should remove a huge profit center from airlines.

Wouldn’t they raise prices to compensate?

Why should you pay more the more you fly, and not just a fixed rate per ton of CO2?

Why do you pay more in taxes on the second dollar you earn compared to the first? It's an acknowledgment that flying is a useful activity for many people, but still an incentive to minimize unnecessary flights.

I think part of the motivation is to not punish infrequent fliers.

I think it's a little like how utilities price water. Water is very inexpensive at first. But if you are filling a pool and watering your lawn a lot, the gallons get more and more expensive.

Where does this carbon "tax" go? For example if the U.S. federal government imposed this tax it's just gets paid to the government to use on whatever?

In Canada, the carbon tax revenue is given back as a rebate which results in a net tax decrease for most families. In the US there have been proposals to use the revenue to cut payroll taxes, I believe Harvard economist (and notably former republican) Greg Mankiw has been pushing this idea for years now.

Where the tax revenue goes isn't the biggest deal since getting a price on carbon alone will result in shifting incentives towards lower carbon behaviors and investments. In my experience watching debates about Carbon Taxes where I live, generally speaking, when people complain about where the money is going they are not arguing in good faith and trying to just spread FUD about the policy to prevent a carbon tax.

Thanks for the first part of your response. Those uses seem potentially interesting.

The second part of your response is somewhat frustrating though. It absolutely does matter what the tax is used for. This can’t just be an excuse for the total tax that I pay to be increased with the additional funds being put towards things like war fighting in far off lands. That’s maybe not as much of a problem in Canada as it is in the U.S. It’s unreasonable to dismiss any concerns about how the tax revenue is used as FUD.

I personally would like to see it go to public universities to fund basic research in areas related to climate change.

Some of it should but since Carbon taxes could be regessive some type of rebate to people on the lower end of the income scale would be good also.

Its important to remember the time constraint in these conversations. Engineering is about constraints, and if you simply remove the biggest one (time) it becomes facile and easy to endlessly talk about future, hypothetical, optimistic "someday" solutions. It is vapid empty happy-talk.

When you factor in the time constraint, specifically the timeframes outlined in the IPCC SR 1.5 report, then you simply cannot claim that direct air capture fuels are a meaningful part of the conversation of what to do with air travels footprint.

I am 100% in favor of continued and increased investment in R&D for DAC. I am 0% delusional that it will be a viable solution in the timeframe we need it to be (same goes for thorium and fusion).

If you're not solving for the time constraint and the curve shape in this graph, you're not talking about "solutions" you're just chit chatting about (cool) tech: https://www.ipcc.ch/site/assets/uploads/sites/2/2019/02/SPM3...

Airplanes are not going to stop flying. We can try raising the price to make them fly less than they would otherwise but there will still be a need to lower the carbon footprint of flight. Synthesizing fuel from captured CO2 seems like a much better pathway than batteries. Also, I don't think its valid to compare it to thorium or fusion. We can make synthetic fuel from captured CO2 today whereas those are science projects. Synthetic fuel isn't cost effective given economic and political realities but its likely this will occur within the next 10 years in at least some places.

Minor point, people seem to marry DAC with synthesizing fuel, probably because Carbon Engineering has gotten a lot of press. Fuel can be synthesized from CO2 from any source. It is my understanding that DAC is currently more expensive than capturing from a point source, like a natural gas power plant. This is due to CO2 being such a small percentage of the atmosphere vs 10%+ from plant flue gases. So, while DAC is more expensive and there are still substantial point sources of CO2, fuel made from DAC will be more expensive from point source capture.

cagenut 5 days ago [flagged]

point source capturing co2 from burning fossil fuels and then turning it into more liquid fuels that you then burn again is madness. you get that right? it makes less sense than simply saying "fuck it" and pumping the same amount of fossil fuel up. at least then you don't have to pay the energy cost of the transitions.

this kind of high-anxiety circular reasoning is hopefully the 'bargaining' phase before you move onto acceptance.

> point source capturing co2 from burning fossil fuels and then turning it into more liquid fuels that you then burn again is madness. you get that right?

Only if you equally value energy at the time of burning and the time of conversion.

Imagine a natural gas peaker plant that captures CO2 during its typical operation, but it converts that CO2 back into a synthetic fuel during periods of energy surplus (when renewables provide more than 100% of the demand). The synthetic fuel is then essentially a battery, which just happens to be energy-dense.

There also reasonable ways of acquiring a "green" CO2 stream that does not involve fossil fuels; non-liquid biofuels (ex, cellulose or charcoal) would work here.

I don't get that because its not true. In both scenarios after the energy consuming activities are done the same amount of CO2 is in the air (ignoring operational efficiency). One just costs more than the other because a more difficult path of capturing the CO2 is chosen.

oh well if we just ignore efficiency and cost!

you are living in a world of spherical cows, not in the reality of the climate emergency.

please understand that as the body counts rise this level of "technically correct" pedantry will be seen for the rather gross complicity it is.

The operational efficiency of both is probably within 10% of each other, so not that significant compared to the big picture of what we are discussing. My guess is that DAC actually fares worse because you have to run an enormous bank of fans to suck in all that air which is why it costs more.

As for cost, I'm not ignoring it, it was my point. Fuel from one pathway costs more from another pathway. High cost is a barrier to taking a solution out of the lab and actually putting it into action to causing less CO2 to be emitted. If 2 fuel pathways have the same carbon footprint but one costs less than the other why would you choose the one that costs more?

I'm willing to listen if you would like to explain why you think DAC is better but so far you haven't done that, you've just been insulting which I don't get.

cagenut 4 days ago [flagged]

I have no idea how you got it in your head that i'm advocating for DAC. I not only did not do that, I specifically did the opposite by saying I have "0%" hope for it. The fact that you've twisted this around now into trying to get me to defend it is an example of why i'm being insulting to you. Your behavior is bad, and I hope you feel bad about it. Why should I treat you with respect when you're ignoring the context of the conversation in order to correct internet strangers with your tangential distraction of a point? What makes you feel entitled to other people being graciously receptive to that?

The actual topic at hand here is the GHG footprint of air travel (see topic, OP). My comment was about the time constraint provided by the IPCC that people fail to include in their analysis. Here you are 5 replies deep twisting my arguments into their opposite while failing to even address, let alone refute or propose your own alternative for the central point.

On a micro level your behavior is just garden variety piss poor social skills (can't say i'm the best either!). On a macro (and specific to this topic, climate change) level your behavior is the sand in the gears that keeps any of the rest of us from discussing this challenge in good faith and reaching rational conclusions. You are poisoning the well. You are pissing in the pool. Please stop.

Personal attacks and flamewar like this will get you banned here. Please review https://news.ycombinator.com/newsguidelines.html and don't do this on HN again.

hi dang

which side are you on?


the bodies are piling up. are you a polite german or a rude one?

i'm a rude one.

If your internet comments are coming from a noble place, you should be able to make them without taking cheap shots or attacking people personally.

you have made a category error if you truly believe that

> I have no idea how you got it in your head that i'm advocating for DAC

I did not say you advocated for DAC. I said I was willing to listen to you explain why you thought DAC was better than point source capture. You said point source capture was "madness" while on the other hand saying DAC research was something you'd be willing to fund. I didn't (and still don't) understand why you think that. Don't think I twisted anything.

You were at angry gibberish in your first reply to me, I tried to give you the benefit of the doubt and merely asked you to explain your position. Obviously, that was a mistake.


Oh I see, thanks for clarifying what you were reacting to. We're not disagreeing, you just misunderstood what I was saying.

Say you have a gas plant that emits 1MT of CO2 in a year. If you capture 1MT of CO2 and then use it to make aviation fuel, then burn that fuel, 1MT of CO2 will have been emitted (as opposed to 2MT if you don't capture and synthesize fuel). This is true whether you capture the CO2 using DAC or if you capture it from the gas plant's flue gases. That's why I said:

> In both scenarios after the energy consuming activities are done the same amount of CO2 is in the air

So, my point was if you wind up with the same amount of CO2 in the air, might as well use the cheaper method to capture the CO2 so that the aviation fuel is cheaper so that it starts getting used sooner.

See, no trolling. I still don't get why you thought I was, it really seems like you have rage issues.

> Say you have a gas plant that emits 1MT of CO2 in a year.

Here you are assuming we don't solve the problem as your first premise. Of course everything downstream of this will be faulty reasoning, because you are defining your start point as having already failed.

> If you capture 1MT of CO2 and then use it to make aviation fuel

Here you are taking your faulty premise and then heaping magic fantasy unicorn technology on top of it. Even if your starting point hadn't been wrong, you fail the time/carbon constraints of SR 1.5 here because this technology simply doesn't exist in a scalable way (and due to physics very likely never will).

This is the practice of climate change denial in 2019. You don't outright deny the problem or the related challenges, you simply spam and ddos the conversation with tangential confused nonsense so that no progress is made.

If you work in some kind of r&d lab on CC tech then by all means, prove me the fuck wrong, PLEASE. If you're just some guy on the internet then please please please wrap your brain around the time-carbon constraints we're solving for and stop arguing for us not to.

I realize it's hard when provoked, but please don't reply to bad comments by breaking the site guidelines yourself. It just makes the thread even worse.


This is an amazing comment that puts it very well into words and sadly it should be added as a disclaimer under about a half of the articles that end up on HN about any future tech.

A quick google suggests that aviation accounts for 2% of global carbon emissions.

Everyone in this thread is pointing out how difficult the engineering is to convert aviation to something emission-free.

To me it seems obvious that we should focus on shrinking the 98% as fast as we can. While we should also work on decarbonizing air travel, I am pretty sure that if we decarbonized everything _else_ that the environmental impact of air (and space!) travel would not be enough to fuel global warming on its own.

Further, because the impact is so much smaller, it might be more feasible for air travel to eventually be required to perform offsetting direct carbon capture than to replace jet fuel with an alternative. That would make flying more expensive, which I think is fine, but it wouldn’t make flying _impossible_ which I think is at least highly undesirable.

To me it seems obvious that we should focus on shrinking the 98% as fast as we can.

The other 98% is made up of lots of industries that account for 1% here, 2% there, 0.1% somewhere else, and so on. We can't afford to ignore something because it's 'only' 2% of the total because that would mean ignoring everything. The reality is that we need to shrink 100% of global carbon emissions, including what's caused by aviation, and everything else.

Shrink everything by 50% would be a more possible goal. While devoting more resources to carbon sequestering.

A quick Wiki suggests that including radiative forcing, the number may be as high as 5%. And given the lack of viable alternative fuels, and the growth of the industry, and potential reduction in other areas, that number may grow to 10, 15 or 25% by 2050, depending on who you listen to.

For individuals who fly, the carbon emissions due to flying makes a non-trivial contribution to their personal carbon budget. As earth becomes more middle class and everybody starts flying, that will become a non-negligible problem.

Nearly every article I've seen on this subject conveniently omits the proportion of aviation's contribution to global carbon emissions.

I'm starting to think it's intentional, and it's instructive to ask why.

Aviation is a topic anyone can understand. It's mostly used as a luxury, so it's easy to justify lowering its usage.

A lot of railway-related lobbying, at least in western europe.

It is 2.8% of global emissions but it is much more if you look only in europe or the US. If I remember correctly it should be around 6% in the EU.

It's even worse for the people who fly, half of the population in America doesn't fly in any one year.

You can make any statistic look worse by arbitrarily restricting the population you apply it to.

It's a tiny fraction of global emissions but, if you fly at all, it's probably a pretty substantial chunk of the emissions you're responsible for. Most people just don't fly and don't need to worry about flight emissions at all.

It's 2% right now, but it will be a larger share as overall emissions shrink. Aviation is so hard to de-carbonize that it was removed from even the non-binding Paris Agreement. We're already working on most of the other 98% but there's not even a vague plan or a goal for reducing emissions from planes yet. That's why it's still news.

Clickbait title. Read the article hoping for great news in the emerging aerospace sectors. What I actually found was a rather large serving of humblebrag with a generous side of monetary privilege.

Based on past decisions, it's going to take some revolutionary technology that is cheaper than avgas fuel/engines. Isn't the USA is the largest and last major country that uses leaded fuel for small planes. We'll do almost anything to save small plane owners the cost of upgrading.

Mainly because avgas is not a meaningful contributor to climate change or environmental lead. It's a pittance when compared to anything else; commercial aviation all sources, DoD, road transport for GhG and piping, paint, and industrial sources for lead.

I should have been clear I was not comparing greenhouse emissions of avgas to replacement but that contributing more lead in any amount to the environment is pretty terrible since we all pay for the effects, even considering whataboutism of all other human lead pollution. The forever persistence of lead and ready bioavailability makes it special.

Leaded avgas is used worldwide wherever high-compression or turbo-charged spark ignition aviation engines operate.

There is an unleaded fuel program (several candidate fuels in testing) and I believe one has a chance to be a drop-in replacement. It’s not yet certified and so we keep using 100LL (worldwide, not just in the US).

>Aviation Is on a Low-Carbon Flight Path

Really? The article just says its author hope for a low-carbon future for the air transport, and mentions some tiny plans by jet makers and early startups to plan to 'hybridify' airplanes.

How much low-carbon would that be? As far as I could read, the author does not even try to evaluate that and blindly hope that it should be helpful, somehow. As if he can't bear flight shame, and tries to share/sell some "hope" to cope with that.

There's always the possibility of hydrogen-powered planes. https://en.wikipedia.org/wiki/Reaction_Engines_A2

Energy density of hydrogen by mass is excellent (about 3x that of gasoline/jet fuel), but much worse by volume (about 1/4 of gasoline/jet fuel).

Finding a way to store liquid hydrogen in 4x the volume (plus any required cooling/insulation and safety systems) might be a challenge for long-haul flights.

Also the production of hydrogen is far from being low-carbon.

Yes there are some ways to produce hydrogen from electricity with electrolysis but at the moment 90% of hydrogen use natural gas or other hydrocarbons and release enormous amount of CO2.

You can say that about everything (except maybe nuclear) right now. This morning I had a discussion with someone on Twitter about windmills, and he said "yeah but they're still manufactured with carbon based processes, cement and steel are made using fossil sources".

Hydrogen production can be clean though.


It can be, but it in fact not. And if it is, at what price?

Don't misunderstand me, I want a low-carbon aviation, I'm all for it. But even if you cut the emissions by a factor 2 of each newer aircrafts, what's the point? The numbers of flights is expected to double in less than 20 years. Global emissions won't reduce at all.


IMHO, we need more regulations (e.g. ban domestics flights if high-speed trains are a viable alternative) rather than technological solutions.

There needs to be some middle ground in these discussions.

Yes, it's silly to ignore realities like "how was the electricity powering this car produced?" OTOH, since we're talking about potential future solutions to a hard problem (hard relative to cars, for now)... you kind of need to dismiss current issues if they're solvable to in theory.

Price of electrolysis hydrogen now, when we're not using that much of it is not necessarily indicative of potential prices at scale.

To me, I think we need to focus regulation that gets high potential technologies (eg electricity production and electric transport, atm) past the point on their learning curves where ordinary price economics and/or bans on carbontech can take over.

The problem with marginal mitigation like discouraging flights or large ICE engines is that gains are one-off, and not en route to bigger solutions.

I agree that commercial flight is a big problem. I'm just skeptical that carbon austerity can amount to more than a rounding error in the long term.

Also a lot harder to handle, and leaks are potentially far more dangerous. Kerosene won't even ignite with a naked flame held to it, whereas H2...

You're correct, but the biggest problem in aviation is weight, volume is a distant second. Large planes can relatively easily get extra volume with a slightly different design, but too much weight just worsen everything.

Increasing volume is not really that easy. You are constrained by aerodynamics and any additional material required to contain an expanded volume adds weight in addition to increasing drag. There is a reason all planes look relatively the same, and it is not because we just happen to think it is a pretty shape.

The problems of using hydrogen are practically non-existent compared to the problems of using batteries. Even if you use half the interior volume of the aircraft for hydrogen storage you are still in the realms of it being economically viable (though still challenging).

1lb of jet fuel has roughly 5kwh of energy. Of course you are going to get less than half of that out as useful work. But an airliner can carry 300000lb of fuel. So as a first order approximation you are going to need 10000+ Tesla battery packs to do the same job.

If you adjust for the drag/energy required to carry more voluminous fuel... any idea how this nets out?

The harder problem is selling hydrogen fuel to consumers.

I was next to a hydrogen refuelling station yesterday at a shell station. It basically looks like they produce it on site with electricity and natural gas... the point is I couldn't order at a drive thru across a parking lot from the thing because it kept making like 15 second long whirlly/screeching pressure sound in some sort of regen cycle.

Fortunately planes don't have much trouble with volume. Taking away a luggage compartment of a long-range airliner like 777 plus it's normal fuel tanks, for hydrogen, will result in at least a trans-atlantic range carbon-free plane. And people already more or less got over the idea of check-in luggage, due to fees.

This is more promising since it is a closed carbon loop and a drop in replacement without capex for retrofits: https://en.wikipedia.org/wiki/Aviation_biofuel

Yep! There's a team working on that in the Netherlands that I'm following: http://aerodelft.nl/project-phoenix/

Hydrogen makes water when burned. Con-trails really need to be investigated for their effects on weather/climate. Coal fired plane might be better since CO2 is a much weaker greenhouse gas than water.

This comment is wrongheaded on several levels.

1: Jet fuel makes water when burned as well: by mass it is ~18% hydrogen, and by energy content it produces 55% as much water as pure hydrogen. The fuel itself is vastly less relevant than the amount burned.

> Coal fired plane might be better since CO2 is a much weaker greenhouse gas than water.

This is a fundamental misunderstanding of how atmospheric water works.

2. Water vapor -which is a gas- is effectively set; it cannot be affected by dropping water in the air. The amount of evaporated water is only affected by atmospheric temperature.

3. Liquid water droplets in the air are just clouds. Unlike water vapor, clouds are extremely good anti-greenhouse gases. They block visible light but allow infrared through. Seeding clouds with sulfuric acid is one of the suggested ways to fight global warming.

4. Contrails are made of ice crystals, not gas or liquid water. Ice clouds cause greenhouse forcing, but the destruction of ozone and other protective gases from high-altitude radicals in jet exhaust is generally considered a larger concern.

5. This is an area of active research, not something that is neglected. All climate research needs a lot more funding.

Con trail are formed when the water from combustion hits the cold surrounding air. They also grow under some conditions, creating more cloud cover.

There are electrification gains to be made on the ground. Aircraft manufacturers are already experimenting with using electric motors for taxiing so that they can wait to start the turbines until close to the runway.


And longer term, electromagnetic catapults embedded in runways can be used to launch aircraft using renewable power. The engineering challenges will be huge and a new generation of airliners will be needed to make it work, but the concept is technically feasible.


I'm curious if anyone here can answer this: what part of flight consumes the most energy/fuel? Is it the take-off? Landing? Cruising? So would a hybrid model plane make sense to use jet fuel or equivalent for the most energy consuming part, then electric batteries for the least intensive parts?

For long-haul, it's cruise. And the energy required can thus be improved directly proportional to lift to drag ratio. What I find promising is work NASA is doing and has done on truss-braced wings to enable a ~doubling of current transonic lift to drag ratios by using an extremely high aspect ratio wing.

take-off uses far more fuel per ground-miles travelled, because in addition to trying to go forward, you're also lifting hundreds of tons of steel, several miles into the air.

but take-off and cruise use probably ROUGHLY the same amount per minute flown - because why waste time cruising at a significantly lower power setting?

> So would a hybrid model plane make sense to use jet fuel or equivalent for the most energy consuming part, then electric batteries for the least intensive parts?

Not at all, because energy is just energy, and full batteries weigh just as much as empty ones, but full fuel tanks weigh a LOT more than empty ones. And empty ones are more efficient to keep aloft.

cite: https://www.usatoday.com/story/travel/columnist/cox/2014/11/...

> but take-off and cruise use probably ROUGHLY the same amount per minute flown - because why waste time cruising at a significantly lower power setting?

Breaking it down in my head, I come to a different conclusion. Engine power during cruise can maintain higher speeds at less power compared to takeoff. The wing generates lift most efficiently at the designed cruise speed when not changing altitude. Lower power at cruise doesn’t mean poor time trade off. Since the power is lower the energy (fuel burnt per minute) should be lower during cruise.

> Engine power during cruise can maintain higher speeds at less power compared to takeoff.

Maintain _which_ higher speeds? The ones that are lower than they could be?

Yes, a plane can generate lift more efficiently when not climbing. But it can generate more speed with more power - so why would it not generate more speed if it has power to spare?

the relationship between fuel efficiency and power isn't linear, especially in turbine engines - the most fuel-expensive per minute periods for passenger planes are taxiing, to the point that there are at least two options on market for electric taxi motors so that engines are started only just before the runway.

Also, you need that power for emergencies. Including separate "maximum power possible" that rips engines apart that is needed on current twin jets.

Also: regardless of acceleration or climb, air resistance is a major factor. This is why most aircraft cruise at 30k-40k feet. Flying the same route and speed at 5k feet burns significantly more fuel.

I think it’s takeoff, regardless of the power plant in the aircraft. Fueled planes are heaviest when taking off, and fighting gravity, and Electric are still fighting gravity.

Why not use renewable energy to synthesize the regular airplane fuel? That would be carbon neutral, and it would avoid all the issues with heavy batteries and upgrading or replacing existing planes.

It likely wouldn't be Jet-A that's synthesized, it would be some sort of bio diesel that would have to get approval. GA still uses leaded gas because the industry is so slow to change and everyone is terrified of having to re-certify (STC) existing aircraft or add additional fueling infrastructure. It's a far more complex issue than I can describe here but it works as a useful analogy.

Electric aircraft, however, sound really good in a boardroom because the logistics are "simpler" (disregarding the issue of charging gigawatt-hour batteries somehow).

What if we work to make air travel less useful. Find reasons why people air travel and eliminate them. For example, conferences where a whole lot of nothing happens - and they 're usually seen as an excuse for mini vacation. Business travel is often a game of persuasion that should be done over skype. Apparently the #1 reason people travel is education & training.

The idea of a battery plane sounds too good to be true , and it 's probably decades away

> What if we work to make air travel less useful.

Air travel is (for a certain distance) the most efficient way to travel. No infrastructure needed along the way (if you discount the air ports used as emergency targets), fast, and quite flexible. Trying to ignore that is futile.

We should simply mandate a growing percentage of carbon neutral fuels for all aircraft traveling to and from our countries. Start with 1% next year, it should be easy to implement and then increase every 2 years by another percentage point.

how is that better than not needing to travel at all?

Because most people like to travel. Any policy that makes people happy is intrinsically better than one that makes them angry or sad. Weird that I have to tell you that.

Because it could actually be done in my estimation?

No discussion of Synthetic Natural Gas => LNG powered jetliners? To me that seems a lot more realistic than banning flight altogether or battery/electric powered aircraft.

My bet is that if you're making synfuels, it makes more sense to take the efficiency hit and go for long chain hydrocarbons instead. Even if LNG is the technically superior fuel, planes last a long time and compatibility with existing equipment is worth a lot.

With the weight of batteries it would definitely be a challenge. Fuel is heavy too, but there is still a factor between their energy density that is close to 100.

Owners of electrical drones know that the fun often ends after around 20 minutes with the battery being the heaviest part by far. I remember there being a drone that could fly for 2 hours with the whole body basically being the battery.

Hmm, when fuel is around 5% of your total ticket cost, something says that there are much more inefficiencies to solve than just the flight itself.

Being forced into security theatre, not allowed your drink or food, forced to overpay in airport... Smells a lot like how housing market is regulated by councils or NY trains having higher carbon emission than Prius due to admin costs...

Most of stewardess could be replaced with a vending machine. Bunk beds made using compliant mechanisms. Tax charge passengers by weight, not by height.

Here's something even more ambitious - it's really bizarre that airlines buy airplanes and then decide to skimp on maintenance. They should really be leasing them from manufacturer who does all the maintenance. Same with pilots, but perhaps they could be airport staff (especially crew). Airline should only focus on ticketing and routing (similar to how energy and telco markets are "deregulated" in some countries).

The planes themselves are also pretty expensive. New Boeing 737 costs $120M. Maintenance also costs money. You need to recover all these costs too.

Is this really more environmentally friendly if the electricity comes from a coal plant driving a turbine? With a gas aircraft, it's still fossil fuels driving a turbine. Sometimes they're very similar turbines: https://www.ge.com/power/gas.

The Carnot efficiency (and joules/“unit pollution”) go up dramatically at scale (one of the reasons the 737 MAX was designed for such huge engines). So yes, a vehicle powered by a coal plant could pollute a lot less than a vehicle burning melted dinosaurs at the point of use.

In addition some of the other pollution (NOx, particulate etc) could be more manageable when coming out of a large smokestack in a desert vs in the cramped spaces of a city (car) or upper atmosphere (plane).

I’m definitely not defending coal! Just saying the calculus is complex and multi factor.

No, of course not, but it allows the opportunity for that same plane to be powered by no-carbon sources in the future. With a gas-engine-powered plane, there's no chance of that.

We just need someone to develop an air-to-air battery replenishment system (same theory/model that you see in use with military tanker aircraft for air-to-air refueling).

I can see it now. Max weight/batteries on take off. Controller to drain batteries serially, and as each one is exhausted, chuck it out the bottom of the plane (with some kind of landing/recovery mechanism). When you start running low on batteries, re-fuel at the nearest tanker.

Obviously I'm kidding. The tanker model is ridiculously expensive. That's why you only see it used for extremely critical things (military).

couple of key points here:

#1 - this article is a great example of how the conversation is shifting, but the mid-point we're at now is a wasteland of nonsense and cognitive dissonance. the text of the article is about the truth of what things matter to reduce your footprint (kids, flying, cars, beef), but the tone of the article and the headline is one of "things are gonna be fine because the whiz kids are working on it" soft-denialism.

#2 - the co2 footprint from jet travel is somewhere between half and 1/4th of the overall greenhouse gas emission footprint of flying. this is because nitrogen oxide and water vapor are also greenhouse gasses (particularly at altitude). It gets very complicated to factor, but at a high level you should simply 3x the carbon cost of flights for the "CO2e" total: https://en.wikipedia.org/wiki/Environmental_impact_of_aviati...

#3 - the electrification of air travel is very well under way. it will not replace trans-oceanic flights in our lifetimes, but it can replace a massive amount of regional travel, and customers can be forced to adapt to a multi-hop world (jfk <-> ord <-> den <-> sfo). If you'd like to learn a ton about the state of the art 1 year ago, check out this playlist from the Sustainable Aviation Summit last year: https://www.youtube.com/watch?v=6LDU0Wgn0Lk&list=PLWUnMAqjJ9...

Given that we already have private jets flying routes that are directly served by airlines non-stop, I doubt how much you can impact business travel.

You can stop kids from seeing their grandparents quite easily by raising ticket prices or taxing jet fuel. I’m not so sure that’s going to keep airlines from serving JFK-SFO direct for business travelers.

you are correct that marginal pricing mechanism are no longer enough to solve this problem. we have moved on to the legislative phase.

I guess I’ll believe it when I see private flights banned or airlines banned from running SFO-JFK direct flights.

I don’t expect to see that in my lifetime (next 40 years).

Yes, the number one problem we have right now is neither technological nor scientific. Its the self defeating circular reasoning of normalcy bias and projecting your unwillingness to change on everyone else.

It costs me $2500 or so to fly to Europe and stay for a week. Maybe $4K in a high week.

I can steer or create a significant multiple of that value by visiting my teams for a week. There’s massive financial incentive for me to take that trip.

yes when the cost of carbon is not accounted for then it creates financial incentives to not just ignore but actively exploit that externality.

Didn't see it listed as an option here, but are the travel-via-ICBM options that SpaceX has been touting going to actually be cleaner options than burning jet-fuel? If you remove the cruise speed requirements for longer-haul flights and put them in space, maybe that helps? Hard to say without any real numbers out there (and even if I had real numbers I'm probably not in a position to synthesize them vs. aircraft)

Judging by the amount of volume of a rocket or ICBM that is consumed by its fuel, I would have to venture that it's nearly impossible that it's cleaner.

I would love to see the numbers for this.

A significant part of the volume in a ballistic rocket is oxidizer which a jet does not need to carry. Liquid methane is much less dense than jet fuel, so it makes sense it would take up much more volume. Also, methane has less carbon in it than jet fuel on a per energy basis.

Still suspect you are right.

As far as spacex visualizes them, no way in hell. Now, if you were to either use A) H2+LOX fuel generated via renewable means or B) non rocket propulsion (scram jet, railgun, etc) you could potentially do a lot better for long flights because once you're in space, there's no air resistance and you can coast as far as you'd like depending on your initial speed.

Probably the best way to cut air travel pollution is with a combination of high-speed electric trains between all major cities (make it convenient to get around without flying) along with a ration book for air travel - everyone is allotted one trip per year, second trip has a surcharge, third and subsequent trips have massive surcharges growing to astronomical numbers.

The average person does not fly in a given year. Those that do fly tend to take no more than one trip. A very small percentage of frequent flyers account for the bulk of air travel. An explicit rationing system would allow "regular" people to still fly occasionally and would have huge benefits for cutting air travel overall while impacting only the frequent flyers. People would be forced to ask if they REALLY need to fly out to NY for that meeting.

The boosts to the VR/Skype/virtual presence/hologram/conferencing field would be enormous.

As a California resident, this reads like a fantasy novel. Sure it would be great to have high-speed trains linking every city, but we cant' even link the two biggest cities in California.

Maybe I'm not as typical as I think, but all your proposal has done is cause me to drive to the one or two conferences I go to each year. Probably not a win at all since I don't drive and EV.

The most interesting part of this article to me is that carbon offsets to cover 12T of CO2 emissions comes out to only $10/month. I've never looked into personal carbon offsets before, but that seems really cheap.

For less than you might pay for a car, or a year of college, or heck, what you might put into your 401k for the year, you can offset effectively your entire life at $16,000. (80y lifespan * 20T/year typical American emissions * $10/ton current price). I am expecting my first child and I've thought about registering for carbon offsets in his name instead of a typical baby registry.

Unfortunately of course, that will only do so much, because the growing global demand for things like air travel, personal car ownership, meat in diets, or air-conditioning mean the numbers on this chart will start to get uglier: https://en.wikipedia.org/wiki/List_of_countries_by_carbon_di...

That's not even the most efficient $/ton offsets that are available. There are non-profits like Cool Earth that are using cleverer approaches to get things down into the vicinity of $1/ton.

The safety benefit of removing the concern of fuel fouling will be a revolution for air travel.

I just hope they don't end up more exposed to bird strike due to bigger more efficient turbines with more composite components.

Some air-breathing batteries can apparently roughly match liquid fuels for specific energy, but they're not rechargeable so much as recyclable, and they also get heavier as they discharge.

As a thought experiment if you could produce jet fuel using carbon captured from the atmosphere (and energy from zero-carbon sources) what other negative externalities would remain from burning it in planes?

I would expect the biggest one to be the opportunity cost of what you could have done instead with that electricity besides make fuel.

For instance, if, say, converting natural gas heating to electric is a better use of energy in terms of reducing greenhouse gasses, then maybe we should do that first.

In the end, a lot of these sorts of proposals depend on cheap renewable energy, and if we can get enough solar and wind power installed then other goals become feasible. Fuel production is an interesting case because you can use surplus energy at times of low demand.

If you mean "what externalities would come from using fuel made using an air-to-fuel process, rather than from oil", then perhaps none.

If you mean "what externalities would come from turning CO2 into fuel and then burning it again to fly an aeroplane, rather than not flying that aeroplane", then that is a complicated and somewhat open question:


Doing my part to solve the issue :)


Hey, nice! What kind of components are you using? Engine, batteries, etc.? Did you pick those yourself, or borrow the design from an existing system like the e-help for hang gliders?

Motor: http://www.freerchobby.cc/ 202/80 27 brushless motor with hall sensor

Controller: https://kellyev.com/shop/khb/ KHB - High Power Opto-Isolated Brushless Motor Controller With Regen (72V-144V) (400A) * must choose high speed option controller to get more than 2000RPM !

Batteries : 6 * CNHL 8000MAH 22.2V 6S 30C LIPO BATTERY Propeller: Wood 49" Pitch 30

What's the advantage of this over using clean energy sources to synthesize jet fuel?

Electricity == low carbon

Not a good starting point for a discussion.

It actually is. Super easy to clean the grid; many grids are extremely clean already.

Guidelines | FAQ | Support | API | Security | Lists | Bookmarklet | Legal | Apply to YC | Contact